Antibodies are immunological proteins that bind to a specific antigen. In most animals, including humans and mice, antibodies are constructed from paired heavy and light polypeptide chains. Each chain is made up of two distinct regions, referred to as the variable and constant regions. The light and heavy chain variable regions show significant sequence diversity between antibodies, and are responsible for binding the target antigen. The constant regions show less sequence diversity, and are responsible for binding a number of natural proteins to elicit important biochemical events. In humans there are five different classes of antibodies including IgA (which includes subclasses IgA1 and IgA2), IgD, IgE, IgG (which includes subclasses IgG1, IgG2, IgG3, and IgG4), and IgM. The distinguishing feature between these antibody classes is their constant regions.
IgG antibodies are most abundantly present in serum, protect the body from the invasion of pathogens, hasten the recruitment of immune system components, and mediate inflammatory responses in tissues. IgG antibodies are tetrameric proteins composed of two heavy chains and two light chains. The IgG heavy chain is composed of four immunoglobulin domains linked from N- to C-terminus in the order VH-CH1-CH2-CH3, referring to the heavy chain variable domain (VH), heavy chain constant domain 1 (CH1), heavy chain constant domain 2 (CH2), and heavy chain constant domain 3 (CH3), respectively.
The IgG light chain is composed of two immunoglobulin domains linked from N- to C-terminus in the order of VL-CL, referring to the light chain variable domain (VL) and the light chain constant domain (CL), respectively.
Under normal conditions, the half-life of most IgG excluding IgG3 isotype in serum is about 22-23 days in humans, which is a prolonged period relative to the serum half-life of other plasma proteins. With respect to this prolonged serum half-life of IgG, IgG that entered cells by endocytosis can strongly bind to neonatal Fc receptor (FcRn, a kind of Fc gamma receptor) in endosomes at a pH of 6.0 to avoid the degradative lysosomal pathway. When the IgG-FcRn complex cycles to the plasma membrane, IgG dissociates rapidly from FcRn in the bloodstream at slightly basic pH (˜7.4). By this receptor-mediated recycling mechanism, FcRn effectively rescues the IgG from degradation in lysosomes, thereby prolonging the half-life of IgG (Roopenian et al. J. Immunol. 170:3528, 2003).
FcRn was identified in the neonatal rat gut, where it functions to mediate the absorption of IgG antibody from the mother's milk and facilitates its transport to the circulatory system. FcRn has also been isolated from human placenta, where it mediates absorption and transport of maternal IgG to the fetal circulation. In adults, FcRn is expressed in a number of tissues, including epithelial tissues of the lung, intestine, kidney, as well as nasal, vaginal, and biliary tree surfaces.
FcRn is a non-covalent heterodimer that typically resides in the endosomes of endothelial and epithelial cells. FcRn is a membrane bound receptor having three heavy chain alpha domains (α1, α2 and α3) and a single soluble light chain β2-microglobulin (β2m) domain. Structurally, it belongs to a family of major histocompatibility complex class 1 molecules that have β2m as a common light chain. The FcRn chain has a molecular weight of about 46 kD and is composed of an ectodomain containing the α1, α2, and α3 heavy chain domains and a β2m light chain domain and having a single sugar chain, a single-pass transmembrane, and a relatively short cytoplasmic tail (Burmeister et al. Nature 372:366, 1994.).
In order to study the contributions of FcRn to IgG homeostasis, mice have been engineered so that at least part of the genes encoding β2m and FcRn heavy chains have been “knocked out” so that these proteins are not expressed. In these mice, the serum half-life and concentrations of IgG were dramatically reduced (Junghans et al, Proc. Natl. Acad. Sci. 93:5512, 1996), suggesting a FcRn-dependent mechanism for IgG homeostasis. It has also been suggested that anti-human FcRn antibodies may be generated in these FcRn knockout mice and that these antibodies may prevent the binding of IgG to FcRn. However, such antibodies have not been generated or tested (WO 02/43658 A).
The inhibition of IgG binding to FcRn negatively alters IgG serum half-life by preventing IgG recycling, so that autoimmune diseases caused by auto-antibodies can be treated. This possibility was shown in a mouse model of autoimmune cutaneous bullous diseases (Li et al. J. Clin. Invest. 115:3440, 2005). Accordingly, agents that block or antagonize the binding of IgG to FcRn may be used in a method for treating or preventing autoimmune and inflammatory diseases, which are mediated by IgG.
Autoimmune diseases and alloimmune diseases are mediated by pathogenic antibodies, and typical examples thereof include immune neutropenia, Guillain-Barré syndrome, epilepsy, autoimmune encephalitis, Isaac's syndrome, nevus syndrome, pemphigus vulgaris, Pemphigus foliaceus, Bullous pemphigoid, epidermolysis bullosa acquisita, pemphigoid gestationis, mucous membrane pemphigoid, antiphospholipid syndrome, autoimmune anemia, autoimmune Grave's disease, Goodpasture's syndrome, myasthenia gravis, multiple sclerosis, rheumatoid arthritis, lupus, and idiopathic thrombocytopenic purpura (hereinafter referred to as ITP). ITP is a disease caused by the destruction of peripheral platelets due to the generation of auto-antibodies that bind to a specific platelet membrane glycoprotein. Anti-platelet antibodies opsonize platelets and result in rapid platelet destruction by reticular cells (e.g., macrophages).
In general, attempts to treat ITP include suppressing the immune system, and consequently causing an increase in platelet levels. ITP affects women more frequently than men, and more common in children than adults. The incidence is 1 out of 10,000 people. Chronic ITP is one of the major blood disorders in both adults and children. It is a source of significant hospitalization and treatment cost at specialized hematological departments in the US and around the world. Each year there are approximately 20,000 new cases in the US, and the cost for ITP care and special therapy is extremely high. Most children with ITP have a very low platelet count that causes sudden bleeding, with typical symptoms including bruises, small red dots on the skin, nosebleeds and bleeding gums. Although children can sometimes recover with no treatment, many doctors recommend careful observation and mitigation of bleeding and treatment with intravenous infusions of gamma globulin.
Methods of treating autoimmune diseases by intravenous administration of IgG (IVIG) in large amounts have been widely used (Arnson autoimmunity 42:553 (2009)). IVIG effects are explained by various mechanisms, but are also explained by a mechanism that increases the clearance of pathogenic antibodies by competition with endogenous IgG for FcRn. Intravenous administration of human immunoglobulin (IVIG) in large amounts has been shown to increase platelet counts in children afflicted with immune ITP, and IVIG has shown to be beneficial as a treatment for several other autoimmune conditions. Many studies have investigated the mechanisms by which IVIG achieves effects in the treatment of autoimmune diseases. With regard to ITP, early investigations led to the conclusion that IVIG effects are mainly due to blockade of the Fc receptors responsible for phagocytosis of antibody-opsonized platelets. Subsequent studies showed that Fc-depleted IVIG preparations provided increases in platelet counts in some patients with ITP, and recently it was reported that IVIG effects are due to stimulation of FcγRIIb expression on macrophage cells, leading to inhibition of platelet phagocytosis.
However, such IVIG treatments have substantial side effects and are very costly to administer. Further, other therapies used for the treatment of autoimmune/alloimmune conditions other than IVIG include polyclonal anti-D immunoglobulin, corticosteroids, immuno-suppressants (including chemotherapeutics), cytokines, plasmapheresis, extracorporeal antibody adsorption (e.g., using Prosorba columns), surgical interventions such as splenectomy, and others. However, like IVIG, these therapies are also complicated by incomplete efficacy and high cost. Also, very high doses of IVIG are required to produce substantial increases in the clearance of pathogenic antibody due to the putative mechanism of IVIG inhibition of FcRn binding with pathogenic antibody (i.e., competitive inhibition) and due to the fact that IgG shows very low affinity for FcRn at physiologic pH (i.e., pH 7.2-7.4), and the typical clinical dose of IVIG is about 2 g/kg.
The use of an inhibitor that competitively inhibits the binding of IgG to FcRn to treat autoimmune diseases is a promising therapeutic method. However, owing to the high affinity of endogenous IgG for FcRn and to the high concentrations of endogenous IgG in blood, it is likely that competitive inhibition of FcRn would require very high doses, and thus have the same limitations similar to those of the current IVIG treatment.
Accordingly, there is an urgent need for the development of a human antibody that has a high affinity for FcRn, and thus can remove pathogenic antibody even at low doses and reduce immunogenicity.